1. Field of the Invention
The present invention is concerned with an improved, tension element anchor apparatus used to restrain retaining walls for earth slopes, excavations and the like, (and a method of installing the same) which is characterized by ease of installation and low utilization expense. More particularly, it is concerned with a tension element anchor apparatus having a screw anchor member connected to an elongated rod by means of a specialized polygonally-shaped drive adapter, and which is designed to be installed in the earth by a screw anchor wrench having one or more elongated, tubular shank sections. Advantageously, multiple shank sections are releasably joined together by a flush-mounted, spring biased dog. A prime feature of the anchor apparatus and method, in accordance with the invention, is that the anchor apparatus may be quickly installed using commonly available equipment producing an anchor apparatus having a consistent load capacity which can be proof-loaded and put into service immediately and yet can be completely recoverable if desired.
2. Description of the Prior Art
Earth-retaining walls, whether temporary or permanent, are used in a wide variety of construction projects, for example in restraining earth slopes, retaining building excavations, and terracing. Essentially, such retaining walls are used to restrain the lateral pressures arising in an adjacent, retained earth mass. The retaining wall usually is oriented in an upright position to effectively restrain these lateral pressures. Although in some limited applications a wedge might be placed in the excavation site to prop up the upright retaining wall, in most applications, it is preferable to secure the retaining wall in some manner to the retained earth mass.
The most widely used present day device for securing a retaining wall to the adjacent earth mass is a combination of an elongated threaded rod encapsulated in cement, where the rod extends laterally through the retaining wall into the earth mass, with one end secured in the earth mass by cement and the exposed end fastened to the retaining wall. Typically, in this type of construction, a lateral hole is first augered in the earth bank to a desired length. Next, depending on the permanency of the anchor, the threaded rod tension element is inserted through a tubular sheathing such that both ends of the rod are exposed. The rod and tubular sheathing is then inserted in the laterally extending hole, leaving one end of the rod exposed at the face of the earth bank. Cement grout is then injected between the rod and sheathing such that the grout is funneled between the rod and sheathing along the length of the rod. At the point where the sheathing terminates, the grout fills the void in the augered hole around the rod, encapsulating the rod end such that, upon hardening, the grout bonds the inboard end of the rod in the earth. After hardening of the grout, the outer face end of the rod is then secured to the wall structure.
Although this grouted rod arrangement is well-established, a myriad of problems are connected therewith, resulting in significant delay and expense. One of the most serious of these difficulties is the necessity to accommodate the curing time of the cement grout. Before the exposed end of the tension element rod can be utilized, a sufficient time must be allowed to enable the grout to cure and harden (seven days is not uncommon). Of course, this period of delay translates into delays for other portions of the construction project, and therefore often represents a major factor in the length of construction time.
A second problem associated with this grouted rod anchor is that once installed, the anchor is extremely difficult to remove from the earth. Often, it is desirable to remove the anchor once its function has been served. For example, in a building excavation it is often desirable to temporarily anchor a retaining wall while the permanent foundation is being constructed. However, the time and expense of excavating the grout and rod from the earth mass dictate that these prior art anchors are more economically abandoned than recovered. In some cases, however, such earth anchors must be recovered to allow for subsequent excavation and construction where the anchor is located, resulting in a major recovery expense.
Added to the significant problems resulting from the use of these prior art anchors are major difficulties associated with the installation thereof. Typically, the drilling of the lateral holes in the retained earth bank produces a significant amount of earth spoils which must be disposed of from the excavation. Additionally, a large void space around the sheathing in the hole must be backfilled in some manner to prevent subsidence of the soil. Further, the equipment necessary to produce and install a grouted rod anchor often consists of several different pieces of specialized machinery, which are expensive to obtain and somewhat cumbersome on the construction site.
In addition to the foregoing problems, there are a number of difficulties associated with the soil mechanics and the construction methodology involved in constructing a sound retaining wall at a minimum cost when using grouted rod anchors. Typically, the first step in constructing a retaining wall is to first determine the retaining capability of the wall necessary to restrain the adjacent earth mass. If the soil composition were uniform along the earth mass, identical grouted rod anchors could simply be implanted with a uniform spacing between the anchors, such that the combined retention capability exceeded the minimum design load strength necessary for the retaining wall. Often, however, the soil composition of the earth mass can vary greatly over the length of the retaining wall, thus varying the anchor retaining capability of the soil in which the anchor is implanted. Because of this soil variability, use of the common grouted rod anchor leaves the engineer with essentially only two choices: first, the soil can be tested along the length of the wall at designated anchor installation points, and the grouted rod anchors constructed to meet the area load requirements of the particular wall section corresponding to these installation points; alternatively, the engineer could assume the worst and install both the quantity and size of anchor necessary on a "worst case" basis. Obviously, both of these options are deficient in terms of the time and expense involved.
Another variable in the effective anchorage of an earth retaining wall is the dynamics of the earth bank adjacent the retaining wall. Generally, the earth mass extending laterally behind the wall is divided into two zones, although the division between the two zones is rather imprecise. First, is an "active" zone immediately adjacent the retaining wall which tends to move with the wall movements and thus is ineffective in restraining the wall. The second zone is an "inactive" zone, adjacent the "active" zone and remote from the retaining wall; the earth in this zone is generally stationary. Therefore, the appropriate locus for implantation of the anchor is the "inactive" zone to assure that the retaining wall is effectively secured.
A further variability in constructing a grouted rod anchor is the inherent installation uncertainty associated with this type of construction. For example, the climatic conditions such as temperature and soil moisture are major factors in the curing time of the grout. Additionally, the angle of the lateral hole and the rod position in the grout bear on the installed strength of the anchor. The installation error, soil variability and the requirement to have the anchor secured in the "inactive" zone, all necessarily require the contractor to proof-load test each anchor to insure that it will meet or exceed the design requirements. However, as may be appreciated, the grouted rod anchor cannot be proof-load tested before the grout has sufficiently hardened. Additionally, as those skilled in the art will appreciate, for a proof-load test to be valid, it should only measure the strength of the anchor in the "inactive" zone. That is, the contractor must take measures to assure that the rod/sheathing arrangement is not restrained during testing by the "active" zone of the earth through which it passes. Therefore, to test the anchor it is necessary to provide the anchor with an "unbonded" section through the "active" zone of the retained earth mass. This "unbonded" section can take many forms, such as a delay in backfill of the hole or a steel tubing telescoped over the sheathing, but in any event further adds to the expense and delay.
It is known in the art to secure an earth retaining wall using a device which includes a screw anchor, but in the known attempt the results have been less than satisfactory. In the known attempt, an earth anchor, having a square, elongated shaft with axially spaced, helical blades secured thereto, was rotatingly driven into a bank of earth leaving the end thereof exposed. The necessary embedment distance of the anchor from the bank face was achieved by successively adding square, elongated shaft sections to the anchor shaft to extend the effective length thereof as embedment proceeded. The final shaft section included a necked-down, threaded reducing end. This reducing end was coupled to an upright wall by means of an appropriate nut threaded onto the reducing end. A significant problem encountered with this type of anchor device and method stems from the fact that, because only the outermost end of the reducing end is threaded, the embedment length of the device is extremely critical. That is to say, if the anchor device is embedded to a point where connection of the threaded end is possible, the device often is not situated for adequate holding in the earth. In this event, it is necessary to either add another section in an attempt to achieve a suitable shaft length or remove the device and reinstall it in another location.